This invention relates to weighing machines using load sensors such as load cells set on a floor to measure the weight of articles such as food items and industrial machine parts. More particularly, the invention relates to methods of and means for correcting the effects of lower-frequency floor vibrations on weight signals generated and processed by such a weighing machine. This invention further relates to a combinational weighing machine adapted to select a preferred combination of scale cells not selected in the preceding cycle of combinational computations for calculating the vertical displacements of the floor.
Floor vibrations usually have lower frequencies than the vibrations caused by an object as it is placed on a weighing machine to be weighed. Thus, it may be thought that a filter can be used to eliminate such lower-frequency vibrations from the weight signals, but the cutoff frequency of the filter must be set low, and this makes the filtering time longer, adversely affecting the speed of weight measurements. U.S. Pat. No. 4,624,331 disclosed a weighing machine having not only a scale cell for measuring the weight of an object and outputting a weight signal indicative of the measured weight, but also a dummy cell set on the same floor as the scale cell such that the effects of the floor vibrations can be eliminated from the weight signal by inverting the dummy signal outputted from the dummy cell and adding it to the weight signal from the scale cell. In this manner, the cutoff frequency of the filter can be set higher, and hence the weighing speed is not reduced.
With a prior art weighing machine of this type, however, the scale cell and the dummy cell must be set close to each other because the floor at the positions of the two cells is assumed to be vibrating under the same conditions. If the conditions of vibrations are different at the positions of the cells, corrections cannot be effected accurately by subtracting the dummy signal of the dummy cell from the weight signal. There may even be situations where the error becomes magnified by the "correction". In short, the choice of the position for installing the dummy cell becomes extremely limited, and the degree of freedom in designing the weighing machine is reduced. If there is not much free space in the neighborhood of the scale cell, in particular, it is extremely difficult to find an adequate place for installing the dummy cell for this purpose. It now goes without saying that the difficulty of this kind is magnified in the case of a combinational weighing machine having a plurality of scale cells and calculating combinations of weight signals therefrom to select a particular combination satisfying a predetermined criterion because each of a large number (such as 10 or 14) of scale cells will have to be provided with a dummy cell associated therewith. Moreover, this has the adverse effects of making the machine large and complicated in structure.
When such a combinational weighing machine with a plurality of weighing units each associated with a scale cell is operated, those of the weighing units which were not selected in the preceding cycle of combinational calculations do not discharge their load and do not receive a new article batch. This means that they are not subjected to a force associated with the loading of a new article batch for the next cycle and hence that the vibration components of their signals are now mainly due to the vibration of the floor on which the weighing machine is installed. For this reason, it has been known in association with combinational weighing machines to make use of the vibration components of weight signals outputted from un-selected weighing units for detecting the mode of vibration of the floor and subtracting the effects of the floor vibration from the measured weight values. With prior art combinational weighing machines of this kind, however, there can be situations where those of the un-selected weighing units used for the detection of floor vibration mode are situated inconveniently with respect to one another such that the area of the portion of the floor formed thereby is small and relatively far from positions of interest. In such a situation, the calculated floor vibration component based on such a small area may be significantly different from the real floor vibration component. In other words, the effects of the floor vibration cannot always be determined accurately or subtracted with a prior art combinational weighing machine of this type.
Japanese Patent Publication Tokkai 64-32122 disclosed a combinational weighing machine adapted to determine the vibration characteristics of the floor from the average of weight signals outputted from those load sensors not selected in the preceding cycle of combinational calculations and to thereby correct the signals outputted from the selected load sensors. This correction routine is based, however, on the assumption that the floor vibrations are identical at the positions of the plurality of load sensors. If this assumption does not hold, the effects of floor vibrations cannot be eliminated accurately by subtracting such an average value.
Another problem with prior art methods using a scale cell in combination with another cell for detecting the floor vibrations relates to the difference in sensitivity between the two cells. In other words, signal levels from these two cells are usually different even if they are subjected to identical loads. This difference in sensitivity is due not only to the material, shape, size and fabrication conditions of the cells (or sensitivity difference characteristically of the cells) but also to the difference in the load. By prior art correction methods by subtracting the signal indicative of the floor vibrations from the weight signal only the sensitivity difference characteristically of the cells (hereinafter referred to as the cell sensitivity) was corrected. This was in part because the weight of the target object, which is to be determined, is a part of the load on the scale cell and is basically an unknown. Since the load on the scale cell may be very different from the load on the vibration-detecting cell, however, prior art correction methods ignoring the sensitivity differences due to difference in load (hereinafter referred to as the weight sensitivity) cannot be accurate. Accuracy in measurements is believed to decline as the weight of the object being weighed increases.
In view of the above, it is an object of the invention to provide a weighing machine capable of accurately removing the effects of floor vibrations from weight signals to thereby yield highly accurate results of weighing.
It is another object of the invention to provide such a weighing machine with an increased degree of structural freedom regarding the positions at which dummy cells can be installed.
It is still another object of the invention to provide such a weighing machine that can be made compact and to cost less.
It is still another object of the invention to provide a weighing machine which does not require any extra dummy cells or amplifiers therefor but is still capable of accurately removing the effects of floor vibrations from weight signals even in situations where the conditions of vibrations are different at the positions of its load sensors.
It is a further object of the invention to provide methods of and means for accurately and quickly removing the effects of floor vibrations on weight values obtained by a weighing machine independent of the size of the weight of the object being weighed.